The invention falls within the field of tire manufacture, and more precisely within the field of the co-extrusion of complex rubber profiled elements.
What is meant by a complex rubber profiled element is a profiled element made up of different profiled elements made from different elastomeric compounds and assembled with one another.
When a green tire is being built, a tread profiled element taking the form of a portion cut from a complex and unvulcanized rubber profiled element is laid on the shaped carcass.
More specifically, and as illustrated in FIG. 1, this portion of complex rubber profiled element 10 includes a transversely continuous sublayer 12 on which another transversely continuous layer 14 is superposed. This other layer 14 is referred to as the tread because it is intended to come into contact with the ground when the tire is running along the ground. Because the sublayer 12 is not intended to come into contact with the ground, it is made from a different elastomeric compound from the tread 14 and does not, for example, have the same composition or the same characteristics and performance.
According to a known method of manufacture, this complex rubber profiled element is obtained by co-extrusion. Using this technique, the various profiled elements of different compounds of the complex profiled element are extruded and assembled simultaneously, on a single manufacturing machine.
Co-extrusion also allows the manufacture of complex profiled elements including a sublayer 12, a tread 14, and other protective profiled elements superposed on the transverse ends of the tread 14 and made from a third elastomeric compound having a composition and characteristics that are different from the characteristics of the sublayer compound and of the tread compound.
However, and like the sublayer 12 and the tread 14, these protective profiled elements are transversely continuous.
Because of the poor electrical conductivity of the elastomeric compounds used for producing the sublayer 12 and the tread 14, co-extrusion machines have been developed that allow the sublayer 12 and the tread 14 to be co-extruded with an insert, made from an electrically conducting elastomeric compound, passing through them.
Such a co-extrusion machine is described, for example in European Patent EP1448355.
According to that document EP1448355, this machine includes a main extruder having an extrusion head with at least two ducts for the flow of a sublayer rubber compound and of a tread rubber compound, the ducts opening onto an extrusion orifice through which the two, sublayer and tread, rubber compounds are discharged, and the extrusion orifice being delimited by a first wall and a second wall.
In order to create an electrically conducting insert through the sublayer and the tread of the co-extruded complex profiled element, the extrusion head also includes at least one micro-extruder of a third, electrically conducting, rubber compound, and the extrusion head of this micro-extruder is equipped at its end with a nozzle, the nozzle passing through the two flow ducts so that the third, electrically conducting, rubber compound is inserted into the sublayer and tread rubber compounds upstream of the extrusion orifice.
A first disadvantage is that the nozzle described in that document EP1448355 is not suitable for extruding an insert made from an elastomeric compound that gives it great stiffness, or at least stiffness that exceeds the stiffness conferred upon each of the sublayer and the tread by their respective compounds. This is because such a compound would need an extrusion pressure that is too high in comparison with the extrusion pressures used to extrude the sublayer and tread compounds, and this could detract from the quality of the co-extruded complex profiled element, notably by leading to the extrusion of an insert that is far wider than needed for electrical conduction or of a cross section that can vary randomly along the length of the profiled element.
Another disadvantage, regardless of whether or not the third compound extruded by this nozzle gives the insert high stiffness, is that this nozzle does not allow the third material to be extruded with a cross section that is precise in the transverse plane of the complex profiled element. Furthermore, neither does this nozzle make it possible to create an insert that is not flanked on either side by the tread and the sublayer.
Now, it so happens that with the increasing use of treads made from elastomeric compounds that give these treads increasingly low stiffnesses and therefore increasingly high hysteresis, there is nowadays within the tire building industry a need to stiffen the complex tread profiled elements via inserts that have a precise cross section in the transverse plane of the complex profiled element, that are not flanked on either side by the tread and the sublayer and that are made from an elastomeric compound that gives them a stiffness higher than the stiffnesses conferred upon the sublayer and the tread by their respective compounds.